Touch screens use a variety of technologies, including resistive, inductive, capacitive, acoustic, piezoelectric, and optical technologies. Such technologies and their application in combination with displays to provide interactive control of a processor and software programs are well known in the art. Capacitive touch-screens are of at least two different types: self-capacitive and mutual-capacitive. Self-capacitive touch-screens employ an array of transparent electrodes, each of which in combination with a touching device (e.g. a finger or conductive stylus) forms a temporary capacitor whose capacitance is detected. Mutual-capacitive touch-screens can employ an array of transparent electrode pairs that form capacitors whose capacitance is affected by a conductive touching device. In either case, each capacitor in the array is tested to detect a touch and the physical location of the touch-detecting electrode in the touch-screen corresponds to the location of the touch. For example, U.S. Pat. No. 7,663,607 discloses a multipoint touch-screen having a transparent capacitive sense medium configured to detect multiple touches or near touches that occur at the same time and at distinct locations in the plane of the touch panel and to produce distinct signals representative of the location of the touches on the plane of the touch panel for each of the multiple touches. The disclosure teaches both self- and mutual-capacitive touch-screens.
Referring to FIG. 10, a capacitive touch-screen device 5 includes a substrate 10. Substrate 10 is typically a dielectric material such as glass or plastic with two opposing flat and parallel sides. An array of drive electrodes 30 is formed on one side of substrate 10 and an array of sense electrodes 20 is formed on the other opposing side of substrate 10. Drive electrodes 30 extend in a drive electrode direction 32 and sense electrodes 20 extend in a sense electrode direction 22. The extent of the drive electrodes 30 and the sense electrodes 20 define a touch-detection area 70. Each location at which the drive electrode 30 and the sense electrode 20 overlap forms a capacitor at touch location 60 at which a touch is sensed; for example touch location 60 is shown in FIG. 6 as a projection from substrate 10 where drive electrode 30 and sense electrode 20 overlap. Thus, touch locations 60 form a two-dimensional array of capacitors corresponding to the locations at which drive electrodes 30 and sense electrodes 20 overlap. Alternatively, each location at which a drive electrode 30 is adjacent to the sense electrode 20 forms the touch location 60 at which a touch is sensed, for example in an embodiment in which drive electrodes 30 and sense electrodes 20 are formed in a common plane (not shown). Touch locations 60 can be associated with specific capacitor locations, as illustrated, or can be interpolated between capacitor locations. A cover 12 (not shown in FIG. 10) is located over substrate 10 to protect sense and drive electrodes 20, 30.
Each of drive electrodes 30 is connected by a wire 50 to a drive-electrode circuit 44 in a touch-detection circuit 40 such as a touch-screen controller. Likewise, each of sense electrodes 20 is connected by the wire 50 to a sense-electrode circuit 42 in touch-detection circuit 40. Under the control of a control circuit 46, drive-electrode circuit 44 provides current to drive electrodes 30, creating an electrical field.
Under the control of control circuit 46, a sense-electrode circuit 42 senses the capacitance of the electrical field at each sense electrode 20, for example by measuring the electrical field capacitance. In typical capacitive touch-screen devices 5, each drive electrode 30 is stimulated in turn and, while each drive electrode 30 is stimulated, the capacitance at each sense electrode 20 is measured, thus providing a measure of the capacitance at each touch location 60 where the drive electrode 30 overlaps the sense electrode 20. Thus, the capacitance is sensed at each touch location 60 in the array of touch locations 60. The capacitance at each touch location 60 is sensed periodically, for example ten times, one hundred times, or one thousand times per second. Changes or differences in the sensed capacitance at the touch location 60 indicate the presence of a touch, for example by a finger, at that touch location 60.
A variety of calibration and control techniques for capacitive touch screens are taught in the prior art. U.S. Patent Application Publication No. 2011/0248955 discloses a touch detection method and circuit for capacitive touch panels. The touch detection method for capacitive touch panels includes scanning the rows and columns of the capacitive matrix of a touch panel respectively, wherein during the scanning of the rows or columns of the capacitive matrix of the touch panel, two rows or columns are synchronously scanned at the same time to obtain the capacitance differential value between the two rows or columns, or one row or column is scanned at the same time to obtain the capacitance differential value between the row or column and a reference capacitance; and then processing the obtained capacitance differential value.
U.S. Patent Application Publication No. 2010/0244859 teaches a capacitance measuring system including analog-digital calibration circuitry that subtracts baseline capacitance measurements from touch-induced capacitance measurements to produce capacitance change values.
U.S. Pat. No. 8,040,142 discloses touch detection techniques for capacitive touch sense systems that include measuring a capacitance value of a capacitance sensor within a capacitance sense interface to produce a measured capacitance value. The measured capacitance value is analyzed to determine a baseline capacitance value for the capacitance sensor. The baseline capacitance value is updated based at least in part upon a weighted moving average of the measured capacitance value. The measured capacitance value is analyzed to determine whether the capacitance sensor was activated during a startup phase and to adjust the baseline capacitance value in response to determining that the capacitance sensor was activated during the startup phase.
U.S. Patent Application Publication No. 2012/0043976 teaches a technique for recognizing and rejecting false activation events related to a capacitance sense interface that includes measuring a capacitance value of a capacitance sense element. The measured capacitance value is analyzed to determine a baseline capacitance value for the capacitance sensor. The capacitance sense interface monitors a rate of change of the measured capacitance values and rejects an activation of the capacitance sense element as a non-touch event when the rate of change of the measured capacitance values have a magnitude greater than a threshold value, indicative of a maximum rate of change of a touch event.
Force sensing is also known in the art. U.S. Patent Application Publication 2011/0227872 discloses a touchpad with capacitive force sensing and can offer tactile feedback to a user's finger, providing a haptic device. This design detects the z (height) position of a finger as well as the x/y position (touch location). A pair of resistance planes are spaced-apart with a resistance mechanism (e.g. springs) to provide mechanical pressure to a finger. One or more force-sensing capacitive circuits are located under the touch surface. Touch location can be determined by the relative force provided by a touch to each of four corners of the touch area or by providing multiple capacitive sensors spatially distributed under the touch surface. Such a design requires the use of carefully controlled and calibrated springs and is thicker than desired.
U.S. Patent Application Publication 2013/0082970, the disclosure of which is incorporated by reference, discloses a positional touch sensor with force measurement. A force-sensing element is disposed with the transparent touch-sensing element and includes two spaced-apart parallel sets of micromesh bands separated by a layer of pressure-responsive material between the micromesh bands forming force-sensing elements. By applying force to the micromesh bands and the pressure-response material and electronically monitoring the force-sensing elements, the allocation and amount of force can be measured. However, such a design requires additional layers of material, reducing transparency and increasing thickness of the touch-sensing device.
Touch-screens, including very fine patterns of conductive elements, such as metal wires or conductive traces are known. For example, U.S. Patent Publication No. 2011/0007011 teaches a capacitive touch screen with a mesh electrode, as does U.S. Patent Publication No. 2010/0026664. U.S. Patent Application Publication No. 2011/0291966 discloses an array of diamond-shaped micro-wire structures.
Although a variety of capacitive touch-and-force-sensing devices are known, there remains a need for further improvements in transparency and thickness.